FIG. 8a illustrates a perspective view of an example of a circularly polarized antenna structure, and FIG. 8b illustrates a schematic sectional view of the circularly polarized antenna structure shown in FIG. 8a (see, for example, Patent Document 1). This circularly polarized antenna structure 30 includes a dielectric substrate 31. An emitting electrode 32 for generating circularly polarized waves is formed on a front surface of the dielectric substrate 31 and a ground electrode 33 is formed on a back surface of the dielectric substrate 31 so as to cover substantially the entire area thereof. An electrode-free region through which a feeding pin 34 is inserted is formed in the ground electrode 33, and the feeding pin 34 is inserted into the dielectric substrate 31 through the electrode-free region. The feeding pin 34 is electromagnetically coupled to the emitting electrode 32 via a capacitance. The feeding pin 34 is connected to an internal conductor of a feeding coaxial cable so that the feeding pin 34 is connected to, for example, a high-frequency radio communication circuit (not shown) included in a radio communication apparatus via the feeding coaxial cable.
In this circularly polarized antenna structure 30, when, for example, a transmission signal is supplied from the high-frequency radio communication circuit in the radio communication apparatus to the feeding pin 34 via the feeding coaxial cable, the transmission signal is transmitted from the feeding pin 34 to the emitting electrode 32 due to the electromagnetic coupling therebetween. Accordingly, the emitting electrode 32 is excited and circularly polarized waves are generated, so that the signal is wirelessly transmitted.
FIG. 9a illustrates a schematic plan view of another example of a circularly polarized antenna structure, and FIG. 9b illustrates a schematic sectional view of FIG. 9a taken along line A-A (see, for example, Patent Document 2). This circularly polarized antenna structure 36 includes a dielectric substrate 37. An emitting electrode 38 for generating circularly polarized waves and a feeding electrode 39 that extends from the emitting electrode 38 are formed on a front surface of the dielectric substrate 37. In addition, a signal line 40, which is a coplanar line (CPW line), is formed on a back surface of the dielectric substrate 37 so as to extend from an edge of the back surface of the dielectric substrate 37 to a position where the signal line 40 faces the feeding electrode 39. In addition, a ground electrode 41 is formed on the back surface of the dielectric substrate 37 such that the ground electrode 41 covers substantially the entire area excluding the region where the signal line 40 is formed and a gap is provided between the ground electrode 41 and the signal line 40.
The coplanar signal line 40 is electromagnetically coupled to the feeding electrode 39. In addition, the signal line 40 is connected to a high-frequency radio communication circuit (not shown) included in a radio communication apparatus. When a transmission signal is supplied from the high-frequency circuit to the signal line 40, the transmission signal is transmitted from the signal line 40 to the feeding electrode 39 due to the electromagnetic coupling between the signal line 40 and the feeding electrode 39, and is then transmitted from the feeding electrode 39 to the emitting electrode 38. Accordingly, the emitting electrode 38 is excited and circularly polarized waves are generated, so that the transmission signal is wirelessly transmitted.
FIG. 10a illustrates a schematic plan view of another example of a circularly polarized antenna structure, and FIG. 10b illustrates a schematic sectional view of FIG. 10a taken along line B-B (see, for example, Patent Document 3). This circularly polarized antenna structure 43 includes a dielectric substrate 44. An emitting electrode 45 for generating circularly polarized waves is formed on a front surface of the dielectric substrate 44. A feeding electrode 46 is formed on a back surface of the dielectric substrate 44 so as to extend from an edge of the back surface of the dielectric substrate 44 to a center position of the emitting electrode 45 on the back surface of the dielectric substrate 44. In addition, a ground electrode 47 is formed on the back surface of the dielectric substrate 44 such that the ground electrode 47 covers substantially the entire area of the back surface of the dielectric substrate 44 excluding the region where the feeding electrode 46 is formed and a gap is provided between the feeding electrode 46 and the ground electrode 47.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-32014    Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-93330    Patent Document 3: Japanese Patent No. 3002252    Patent Document 4: Japanese Unexamined Patent Application Publication No. 1-147905
In the antenna structure 30 shown in FIGS. 8a and 8b, the feeding pin 34 is used. Therefore, it is necessary to insert the feeding pin 34 into the dielectric substrate 31 after the emitting electrode 32 and the ground electrode 33 are formed on the dielectric substrate 31 in the manufacturing process, and thus the manufacturing process is complex. In addition, in the antenna structure 30, the emitting electrode 32 and the feeding pin 34 are preferably electromagnetically coupled to each other while impedance matching is obtained. To provide impedance matching between the emitting electrode 32 and the feeding pin 34, an end of the feeding pin 34 must be precisely positioned with respect to the emitting electrode 32 so that the distance between the emitting electrode 32 and the feeding pin 34 is set to a predetermined distance for impedance matching. However, in mass production, for example, it is extremely difficult to insert the feeding pin 34 into the dielectric substrate 31 as designed in all of the products. Therefore, the distance between the emitting electrode 32 and the feeding pin 34 varies depending on the product and the condition of impedance matching between the emitting electrode 32 and the feeding pin 34 varies accordingly. Since the radio communication performance varies depending on the condition of impedance matching between the emitting electrode 32 and the feeding pin 34, reliability of performance cannot be ensured.
In addition, the antenna structure 30 is connected to the high-frequency radio communication circuit in the radio communication apparatus using the coaxial cable. Therefore, there are problems that a cumbersome task of connecting the coaxial cable to the antenna structure 30 is necessary and the manufacturing cost is increased.
In the antenna structure 36 shown in FIGS. 9a and 9b, not only the emitting electrode 38 but also the feeding electrode 39 is formed on the front surface of the dielectric substrate 37. Since the feeding electrode 39 must be formed, it is difficult to reduce the size of the dielectric substrate 37.
In the antenna structure 43 shown in FIGS. 10a and 10b, the feeding electrode 46 is formed so as to extend from the edge of the back surface of the dielectric substrate 44 to the center position of the emitting electrode 45 on the back surface of the dielectric substrate 44. Therefore, there is a problem that satisfactory resonance for generating the circularly polarized waves cannot be obtained by the emitting electrode 45 because of the reason described below and it is difficult for the antenna structure 43 to function as a circularly polarized antenna.
A current (resonance current) that flows in the emitting electrode 45 travels along linear paths that pass through the center O of the emitting electrode 45, for example, along paths shown by dashed lines α and α′ in a plan view of FIG. 10c. Accordingly, an image current that is induced by the resonance current in the emitting electrode 45 and that flows in the ground electrode 47 preferably travels along the paths α and α′ of the resonance current in the emitting electrode 45, that is, the linear paths that pass through the center position O of the emitting electrode 45. However, since the feeding electrode 46 formed on the back surface of the dielectric substrate 44 extends to the center position O of the emitting electrode 45 and the ground electrode 47 is not formed in a region around the center position O of the emitting electrode 45, the image current in the ground electrode 47 travels along paths that go around the feeding electrode 46, as shown by solid lines β and β′ in FIG. 10c. More specifically, unlike the resonance current in the emitting electrode 45, the image current cannot travel along the linear paths that pass through the center position O of the emitting electrode 45. Therefore, the length of the paths along which the image current travels is longer than the length of the paths along which the resonance current travels in the emitting electrode 45. For this reason, satisfactory resonance for generating the circularly polarized waves cannot be obtained by the emitting electrode 45.